The measurement of pH in saline and hypersaline media at sub-zero temperatures: Characterization of Tris buffers

摘要

The pH on the total proton scale of the Tris-HCl buffer system (pH(Tris)) was characterized rigorously with the electrochemical Flamed cell in salinity (S) 35 synthetic seawater and S = 45-100 synthetic seawater-derived brines at 25 and 0 degrees C, as well as at the freezing point of the synthetic solutions (-1.93 degrees C at S = 35 to -6 degrees C at S = 100). The electrochemical characterization of the common equimolal Tris buffer [R-Tris = m(Tris)/m(Tris-H+) = 1.0, with m(Tris) = m(Tris-H+) = 0.04 mol kg(H2O)(-1) = molality of the conjugate acid-base pair of 2-amino-2-hydroxymethyl-1,3-propanediol (Tris)] yielded pH(Tris) values which increased with increasing salinity and decreasing temperature. The electrochemical characterization of a non-equimolal Tris buffer variant (R-Tris = 0.5, with m(Tris) = 0.02 mol kg(H2O)(-1) and MTris-H+ = 0.04 mol kg(H2O)(-1)) yielded pH(Tris) values that were consistently less alkaline by 03 pH unit than those of the equimolal Tris buffer. This is in agreement with the values derived from the stoichiometric equilibrium of the Tris-H+ dissociation reaction, described by the Henderson - Hasselbalch equation, pH(Tris) = pK(Tris)* + logR(Tris), with pK(Tris)* = stoichiometric equilibrium dissociation constant of Tris-H+, equivalent to equimolal pH(Tris). This consistency allows reliable use of other R-Tris variants of the Tris-HCl buffer system within the experimental conditions reported here. The results of this study will facilitate the pH measurement in saline and hypersaline systems at below-zero temperatures, such as sea ice brines. (C) 2016 The Authors. Published by Elsevier B.V.